1. Field of the Invention
This invention relates to closure of electromagnetic devices and more particularly to closure of electromagnetic contactors in which electrical contacts are closed and held closed by controlling application of current to a coil of an electromagnet.
2. Background of Information
Electromagnetic contactors are electrically operated switches used for controlling motors and other types of electrical loads. Contactors include a set of movable electrical contacts which are brought into contact with a set of fixed contacts to close the contactor. The contacts are biased open by a kickout spring. A second spring, called a contact spring, begins to compress as the moving contacts first touch the fixed contacts. The contact spring determines the amount of current that can be carried by the contactor and the amount of contact wear that can be tolerated. The movable contacts are carried by an armature of an electromagnet. Energization of the electromagnet overcomes the spring forces and closes the contacts.
In earlier contactors, the energy applied to a coil of the electromagnet was substantially in excess of that required to effect closure. While it is desirable to have a positive closing to preclude welding of the contacts, the excess energy is unnecessary and even harmful. If the armature of the electromagnet seats while traveling at a high velocity, the excess kinetic energy is absorbed by the mechanical system as shock, noise, heat, vibration and contact bounce.
One type of electromagnetic contactor is disclosed in U.S. Pat. No. 4,893,102. This system reduces contact bounce which may occur when the respective contacts of the electromagnetic contactor impact each other during an actuation cycle. This is achieved by controlling energization of the contactor coil in four separate stages: (1) an acceleration stage; (2) a coast stage; (3) a grab stage; and (4) a hold stage. When at rest, the contacts are held in a normally open position by the force of the kickout spring disposed within the contactor assembly. In the acceleration stage, the contactor coil is fully energized and the contacts are accelerated toward a closed position at a maximum rate. In the coast stage, the contact mechanism has already achieved enough velocity to achieve closure, so energization of the contactor coil is reduced or eliminated entirely to reduce the force of contact closure impact to a minimum level. In the grab stage, the system evaluates a closing velocity of the contactor mechanism and adjusts energization of the contactor coil to ensure the contactor mechanism has enough momentum to guarantee contact closure. Finally, in the hold stage, energization of the contactor coil is reduced to a level sufficient to counteract the force of the kickout spring and maintain the contacts in a closed position.
U.S. Pat. No. 5,128,825 is directed to an electromagnetic contactor which accommodates to dynamic conditions of the contactor coil and supply voltage to provide consistent closure characteristics of low impact velocity and reduced contact bounce of about 6 ms. The contactor gates a first voltage pulse to the coil of the contactor electromagnet at a fixed, preferably full, conduction angle, and monitors the electrical response of the coil, namely the peak current. The conduction angle of the second pulse is then adjusted based upon the peak current produced by the first voltage pulse and the voltage of the first pulse to provide, together with the first voltage pulse, a constant amount of electrical energy to the coil despite variations in coil resistance and supply voltage. The third and subsequent voltage pulses to the coil of the contactor are gated at conduction angles preselected in order that, with constant energy supplied by the first and second voltage pulses, the contacts touch and then seal at a substantially constant point in a selected pulse. Contact closure can occur at the third pulse, or in a large contactor where more energy is required, at a later pulse. Contact touch and sealing consistently occur on declining coil current in order to achieve low impact velocity and reduced contact bounce.
Normally, the third and subsequent pulses are gated to the contactor coil at constant, preselected conduction angles. However, under marginal conditions for closure where the peak current produced by the first voltage pulse is below a predetermined value, a second set of conduction angles is used to gate the third and subsequent voltage pulses to the coil. This second set of conduction angles produces a substantially full conduction of the third and subsequent pulses.
While the microcomputer controlled contactor of U.S. Pat. No. 5,128,825 is a great improvement over earlier contactors, and goes a long way toward providing positive closure with reduced contact bounce by accounting for dynamic changes in the characteristics of the contactor electromagnet, there is room for improvement. Although the volt-amps (VA) required for closure is premeasured and a recipe is predetermined for closing the contactor with low bounce, several limitations include: (1) the recipe is not calculated during operation and, thus, is stored in the limited non-volatile memory of the microcomputer; (2) the recipe covers a wide VA range and, therefore, is not optimized for the very low or the very high ends of the VA range; (3) the recipe provides control without feedback and, hence, abrupt changes in the line voltage and line frequency are not included in the closure control algorithm; and (4) stored recipes require significant digital logic and, thus, additional cost to implement.
There is a need, therefore, for an improved contactor which provides a consistent closing time and a consistent armature closing velocity with minimum contact bounce.
There is a further need for such a contactor which consistently reduces armature closing velocity and, thus, contact bounce time.
There is an additional need for such a contactor which takes into account dynamic changes in line frequency and line voltage.
There is a more particular need for such a contactor which generally operates independently of the line frequency and voltage.